1. Field of Invention
The present invention relates to piezoelectric transducers and more specifically to piezoelectric transducers driven by digital signals thereby eliminating additional analog circuitry in electronic devices.
2. The Prior Art
A piezoelectric element is a crystal which delivers a voltage when mechanical force is applied between its faces, and it deforms mechanically when voltage is applied between its faces. Because of these characteristics a piezoelectric element is capable of acting as both a sensing and a transmitting element. Piezoelectricity exists because some atomic lattice structures have as an essential cell a cubic or rhomboid atomic cage, and this cage holds a semi-mobile ion which has several stable quantum position states inside itself. The ion""s post ion state can be caused to shift by either deforming the cage or by applying an electric field or voltage. The coupling between the central ion and the cage transforms electrical charge to mechanical strain and vice versa.
Based on these fundamental principles of piezoelectricity, it is well known that if an analog signal is applied to a piezoelectric element, the piezoelectric element will deform, thereby creating a sound pressure which in turn may create an audible sound. Piezoelectric speakers can generate a wide range of high sound pressures. In addition, piezoelectric elements may be manufactured using an ultra thin piezoelectric film, which allows the piezoelectric elements to be made quite small. Such piezoelectric elements have been used to provide sound audible in the human hearing range for such devices as speakers for computers, cordless phones, alarm clocks, fire alarms, buzzers, headphones, and earphones driven by analog signals.
Unlike conventional speaker systems, a piezoelectric element is capable of creating a sound without a fragile or moving coil. Piezoelectric elements thus are ideal for today""s electronic devices because they do not emit a significant amount of electrical noise or electromagnetic interference (xe2x80x9cEMIxe2x80x9d). The EMI produced by coil-based speakers is usually lower than the EMI generated by the audio, radio and power supply circuitry. As a more significant advantage, piezoelectric transducers are significantly cheaper than electrodynamic speakers. In addition, piezoelectric elements are compatible with solid state devices because they are rugged, compact, reliable and efficient. A further benefit of piezoelectric elements is that they consume a little amount of power compared to the amount of acoustic pressure they can generate.
A problem with conventional piezoelectric elements is that in order for them to reproduce human voice or music sounds with acceptable quality they have had to be driven by analog voltage signals. See FIGS. 3 and 4, and discussion below. Because piezoelectric elements used to reproduce human voice or musical sounds are driven by analog voltage signals, any system using these devices must incorporate analog circuitry, such as an operational amplifier circuit or a digital to analog converter. Analog circuitry is sensitive to noise and EMI. EMI disturbs electronic equipment, as perceived in the form of undesired audible noises and distortions on the output audio signal of cellular phones and other equipment. An electromagnetic field is a combination of electric and magnetic fields. The frequency of oscillation can range from a fraction of one Hertz (cycle per second) to many million Hertz. EMI will decrease the overall performance and reliability of affected electronic devices using analog circuits.
Currently, cellular phones are designed with analog driven electrodynamic speakers to generate audible sounds. As illustrated by the schematic in FIG. 4, these prior art systems use digital signal processors or microprocessors to drive a digital to analog converter, which in turn, provides an analog signal to the electrodynamic speaker.
Another method of providing sound is depicted in more detail in FIG. 3, wherein a piezoelectric element is driven directly by an analog signal. These piezoelectric systems also use a digital to analog converter to create a usable signal for the piezoelectric transducers. Both of these prior art systems are susceptible to the problems associated with EMI, which is a common source of noise heard in acoustic generation devices.
The EMI which causes unwanted noise in the speakers of modern communication devices, such as cellular phones, is largely generated by the analog and digital circuitry associated with the means for driving said speakers. In cellular communication systems several different mobile units share the same set of frequency channels at the same time. In order to share the same frequency, the mobile units using the system sample a given set of frequency channels (spread spectrum) at a predetermined and controlled rate. The three basic spread spectrum types are code-division multiple access (CDMA), time-vision multiple access (TDMA), and frequency-vision multiple access. Turning the sampling circuitry, which monitors the signals being sent by a base station, on and off at high rates of speed generates EMI. Such EMI can cause problems with cellular phone spear clarity, because it is a source of noise which can be heard by humans on the speaker system. Some say the noise makes the phone sound as if the speaker is making a low hissing or crackling sound (xe2x80x9cmotor-boatingxe2x80x9d).
Thus, a need exists in the electronic industry to replace analog driven speakers in various products, including cellular phones, with purely digitally driven speakers which are less susceptible to EMI. The present invention discloses a digitally driven piezoelectric transducer which is not dependent on analog circuitry to produce audible sound, thereby eliminating the problems with EMI and the need for additional analog circuitry.
The present invention comprises a digitally driven piezoelectric transducer. The invention uses a piezoelectric element having a plurality of electrically isolated conductive sections carried by one side of the piezoelectric element and a conductive common plate carried by the other side of the piezoelectric element. In addition, the invention includes a resonant cavity which is connected with the piezoelectric element which intensifies the sound energy produced by the piezoelectric transducer.
A digitally driven piezoelectric transducer avoids the problems associated with EMI because it eliminates the need for additional analog circuitry to create sound audible to humans. In the invention, a plurality of electrically isolated conductive strips of varying size are carried by one side of a piezoelectric element. The electrically isolated conductive strips may take the form of any convenient shape and are specifically designed to cover areas of predetermined sizes on said piezoelectric element. The electrically isolated conductive strips are integrally formed with the piezoelectric element, covering predetermined surface areas to form a binary progression. Each electrically isolated conductive section is driven by a different bit of the parallel digital signal supplying the acoustic information, and thus generating sounds in different ranges according to the input signals.
The present invention therefore avoids the problems with prior art speaker systems by avoiding the use of digital to analog conversion circuitry. By not using this conversion circuitry, the invention allows designers to avoid using unnecessary analog circuitry that is susceptible to EMI. In addition, since digital piezoelectric transducers have no coils or magnets, they are less likely to pick up EMI radiated by other systems.